Hello Adam369
More info here about the OPI CM4 with a custom dtb file
The PCM5102 just need a i2s bus and i put for my pcm5102a somme connection with the hat
PI0 → I2S SCK–>pin29
PI1 → I2S BCK–>pin12
PI2 → I2S LCK–>pin35
PI3 → I2S DATA–>pin40
5V → to VIN power of DAC board–>pin4
GND → to GND power of DAC board–>pin6
Hope it can help you